Understand Mac IP Settings
Understanding Internet Address Assignment
When you connect to the Internet, you are assigned a unique Public IPv4 address, such as 225.195.118.167, or a newer IPv6 address, like 2000:cfa6:d633:9d1:8aca:d44a:7af8:800. You can verify your address at https://test-ipv6.com/. However, these addresses can be difficult to communicate for those who are not familiar with technical terms, and even specifying MAC addresses like 50:80:b1:80:5c:dc can be error-prone and complex. Furthermore, this method does not provide any historical data.
How the Internet Access is Facilitated
In order to access a website, such as https://little-batz.co, you initially query a DNS server to convert the domain name (little-batz) and the Top Level Domain (co) of the URL into an IP address, such as 170.170.136.103. Every web request from your computer and browser includes detailed information about its type, for example:
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_3) AppleWebKit/537.75.14 (KHTML, like Gecko) Version/7.0.3 Safari/7046A194A
The Significance of Default Gateways
The default gateway is typically an address that is automatically assigned through DHCP. This gateway, like 172.20.207.98 (usually ending in .1 or .254 depending on the scope size), is where your computer forwards all its network traffic. A comprehensive explanation of configuring IPv6 connectivity can be found at how-to-fix-ipv6-connectivity/ but it can also be verified on Mac or Linux using the following command:
ip -6 route show
IPv4 Routes and the Host IPv4 Route Table (inc. VPN)
netstat -rn -f inet | egrep -i "default|0/1|128.0/1"
0/1 172.18.12.193 UGScg utun3 default 172.20.207.98 UGScg en0 128.0/1 172.18.12.193 UGSc utun3
Note: We are not just looking for the default but also for any VPN that overrides the public v4 address space.
IPv6 Routes and the Host IPv6 Route Table (inc. VPN)
netstat -rn -f inet6 | egrep -i "default|2000::/3"
If you have IPv6 active the above should return at least one route (as per below) via a known interface such as “en0 " on a Mac.
default fe80:76dc:236d:1ff9:4c2c%en0 UGcg en0 default fe80::%utun0 UGcIg utun0 default fe80::%utun1 UGcIg utun1 default fe80::%utun2 UGcIg utun2 2000::/3 utun3 USc utun3
Note: We are not just looking for the default but also for any VPN that overrides the public v6 address space.
Debugging DHCP for both IPv4 and IPv6
To get a look at the low level DHCP configuration (Mac/Linux):
ipconfig getpacket en0
...
domain_name_server (ip_mult): {64.234.213.42, 121.64.108.205}
end (none):
...
So, in the above we are not getting IPv6 DNS servers from the DHCPv4 reply but…
ipconfig getv6packet en0
DHCPv6 REPLY (7) Transaction ID 0x80940b Length 76
Options[4] = {
CLIENTID (1) Length 14: DUID LLT HW 1 Time 668691856 Addr 50:80:b1:80:5c:dc
DNS_SERVERS (23) Length 32: 2606:4700:4700::1111, 2001:4860:4860::8844
DOMAIN_LIST (24) Length 0: Invalid
SERVERID (2) Length 10: DUID LL HW 1 Addr 84:f0:a3:83:79:00
}
Solving Connectivity Issues for Wired and Wireless Networks
When it comes to transmitting data to your router, you may be using a wired or wireless (Wi-Fi) medium at the physical and data layer.
Strategies for Apple macOS / OSX Users
Regardless of the version of OSX/macOS you have - whether it’s 10.11.2, 11.2.7, or 12.1.8 - there are various troubleshooting tools available. However, the manual actions and scripts may not provide a series of correlated values over time. This is where automated remote troubleshooting becomes essential, especially for teams that have adopted remote work and Work From Anywhere (WFA) practices.
Useful Built-in Scripts
One of the helpful tools on OSX/macOS is sudo wdutil info, which provides a dump of the current wireless settings to the CLI and can also be configured to generate specific logs for troubleshooting. Additionally, the sysdiagnose tool can be used to generate a wide range of logs, although much of it is only point-in-time in relation to wireless, similar to wdutil.
To run sysdiagnose in the background and write logs to /var/tmp/<blah>.tar.gz, use the command sudo nohup /usr/bin/sysdiagnose -u &. If you prefer to run it interactively, use the command sudo /usr/bin/sysdiagnose, which will give a privacy warning and open Finder in the correct location. Carefully consider the file sizes, which are approximately 300MB.
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